This application is related to an application entitled CMOS IMAGE SENSOR by Michael Guidash filed on the same day as the instant application, and an application entitled SEMICONDUCTOR IMAGE SENSOR AND METHOD THEREFOR by Drowley et al filed concurrently herewith.
This invention relates, in general, to semiconductor devices, and more particularly to a semiconductor image sensor.
In the past, a variety of methods were used to form semiconductor image sensors on a substrate with complementary metal oxide semiconductor (CMOS) devices. Typically, the optical receiving portion of the sensor is formed either as a gate of a large area transistor, often referred to as a photo-gate or as a source-drain junction of a metal oxide semiconductor (MOS) transistor. The photo-gate transistor implementation requires that light travel through the silicon gate of the transistor in order to convert light to electrical energy. Consequently, the photo-gate implementation has reduced sensitivity. Additionally, the depletion region generally is shallow (less than one micron) thereby reducing the collection efficiency of carriers induced by red light absorption. Also conventional photo-gate implementations are susceptible to noise created by surface recombination.
The source-drain junction implementation generally has a junction that is optimized for transistor operation and therefor also has a shallow junction that results in inefficient collection of carriers induced by red light. Another disadvantage of the source-drain junction implementation is that the junction typically is formed in a highly doped (greater than 10.sup.16 atoms/cm.sup.3) region that limits the width of the junction depletion region thereby further reducing the collection efficiency of carriers induced by red light absorption. Furthermore, forming the junction in such a highly doped region results in a large capacitance that reduces the amount of charge that can be transferred from the photo sensing element to other electronics.
Accordingly, it is desirable to have an image sensor that does not utilize a photo-gate thereby resulting in higher efficiency, that does not have a shallow junction depth thereby increasing efficiency, that minimizes noise from surface recombination.